Electrolytic recording



Patented Sept. 26, 1944 ELECTROLYTIC RECORDING Edgar-B. Wagner, NewYork, N. Y., assignor, by mesne assignments, to Faxlmile, Inc., NewYork, N. Y., a corporation of Delaware No Drawing. Application October25, 1940, Serial No. 362,787

4 Claims.

The present invention concerns electrolytic recording and, inparticular, sensitizing materials for facsimile electrolytic recordingsurfaces.

One object of the present invention is to provide sensitizing materialfor a facsimile recording sheet capable of improved image reproduction.

Another object is to provide a sensitized sheet for facsimile or otherrecording capable of yielding a sharper and more permanent print thanhas hitherto been obtainable.

- Still another object is to provide. materials capable of yielding asharp electrolytic print which does not spread or lose its detailsubsequent to recording.

A still further object is to provide recording materials which do notdiscolor subsequent to recording.

These and; other objects will be apparent from the detailed descriptionof :the invention.

Electrolytic recording is the art of recording by means of theelectrolytic action of an electric current upon a properly sensitizedmedium. The recording current is localized to produce marks undercontrol of a machine or operator. In facsimile recording it is commonpractice to scan :.a record sheet with a stylus carrying currentmodulated according to the instantaneous density of the subject to bereproduced. A properly sensitized sheet, for instance, may be passedun-= der a reciprocating stylus carrying the recording current or theequivalent scanning operation may be carried out in a number of wayswell known to those skilled in the art.

In order toproduce a mark or image on the recording sheet the'sheet mustcarry on its surface or impregnated in its structure materials whichchange color or optical density or both in response to the passage on anelectric current.

One method of preparing such a current sensitive surface is toimpregnatea sheet of paper with pyrocaiechin, sodium or potassium nitrate, andwater, and to record by means of an iron stylus carrying the recordingcurrent. The sodium nitrate in water solution is electrolyticallyconductive and is decomposed upon the passage of the recording current.An alkali, sodium hydroxide, is produced at the cathode of the recordingcircuit. This alkali passes thru the paper to the anode where the ironof the recording electrode is dissolved and meeting the pyrocatechin inthe presence of the alkali forms a color-lake. The iron lake is denseand nearly black in color, providing a record of high contrast anddensity. A small amount of an acidstrong enough to dissolve the lake maybe added to sharpen detail and prevent spreading of the recorded image.The acid which prevents lake formation is neutralized by the abovementioned alkali at the recording epoint, allowing the lake to form,producing a desired image, but prevents spreading of the lake tonon-recording points by destroying the lake beyond the point of marking.

It has been found that a second form of spreading and loss of detail inthe recording is due to the fact that the color lake forming the imageis a hydrated colloid, or hydrophyllic sol, and is composed ofpositively charged particles. If enough excess moisture remains afterrecording, or is picked up from a humid atmosphere, these chargedparticles repel each other and tend to migrate, especially when the EUions from the unmarked part of the paper diffuse into the marked zones.

Dehydration suflic'ent to remove this excess moisture, subsequent torecording, is possible and is effective, but is cumbersome and slow. Itwould require a drying attachment to the recorder, which would increaseboth the weight of the instrument and its power consumption.

Migration may also be prevented by removal of the positive charge. Thisdischarge or precipitation may be accomplished by the use of a salt' oracid whose negative ion is of higher valence than the positive. It hasbeen found that an ion with a valence of at least three is highlydesirable, since the trivalent ion is much more effective than thedivalent and the tetravalent than the trivalent. Since it must, ingeneral, also be soluble and colorless, the number of availablesubstances with polyvalent-ions is limited. Phosphates, citrates andnaphthol disulfonates have been found to be particularly desirable.

It has been found that phosphoric acid is effective as a precipitatingelectrolyte when used in an amount as low as two per cent by weight ofthe amount of pyrocatechin used as sensitizer.

This phosphate ion is equally effective whether introduced as aphosphoric acid or as an acid phosphate.

It has been found that citric acid or naphthol disulfonic acid inamounts of the order of 0.5 per cent may also be used as precipitants.

It has been found that since many trivalent ions, such as PO-l, forminsoluble ferric salts, the lake formed in their presence is in muchlower concentration than if the insoluble ferric salts were not present.It has also been found that, in such cases, a satisfactory amount oflake can be obtained if a polyhydroxy compound, such as glycerine,dextrine, mannitol, citric acid, tartaric acid, or a glycol, is alsopresent. The amount of such hydroxy body, which can be effectively used,varies with its nature. For instance, as much as 10 per cent ofglycerine may be used, whereas as little as 0.5 per centof citric acidmay interfere with oxidation of the electrode.

It has been found that while the above materials and methods produce aninitially sharp and contrasty print, the light colored background may besubject to discoloration due to oxidation, especially if the recordingis exposed to sunlight or oxidizing oils, such as linseed and tung oils.

According to the present invention this discoloration by oxidation maybe substantially prevented by the addition of a small amount of apowerful reducing agent to the sensitizing material. Among thosesubstances, which have been found to be effective antioxidants when usedin quantities small enough not to unduly increase the power requirementsof recording, usually less than five per cent of the aromaticlake-forming compound, are salts of hydrazine and phenyl hydrazine, suchas the sulfate or hydrochloride, allyl thiourea, thiourea, and salts ofsemi-carbazide, such as sulfate or hydro-chloride. Thiourea, because ofits effectiveness and commercial availability, has been found to beparticularly useful. Also, a mixture of more than one of these agentshas been found to be more effective than one alone, as, for instance,thiourea and semicarbazide hydrochloride; or a mixture of hydrazinesulfate and allyl thiourea.

By combining the-precipitant and antioxidant in the same formula, theresulting sensitization is capable of giving very sharp detail in aprint that is permanent, and at the same time, paper so prepared resistsdiscoloration on exposure to sunlight, catalysts, and high humidity,over a much longer period of time than papers hitherto available.

A small amount of penetrant may be added to lower the surface tensionduring impregnation. Various compounds may be used for the purpose.Commonly used penetrants are sodium alkyl sulfonate, alkylated arylsulfonate, and dioctylester of sodium sulfo succinic acid.

Specifically, recording papers may be prepared by limpregnation with anyof the following formu ae:

Water cubic centimeters 100 Potassium nitrate grams 20 Pyrocatechin do 5NaH2PO4'H2O d 0.3 H3PO4(35% strength) cubic centimeter 0.05 Thioureagrams-.. 0.1 Penetrant cubic centimeters 0.1

Water "cubic centimeters 100 Potassium nitrate grams 20 Pyrocatechin doH3PO4 cubic centimeters 0.1 Thiourea. grams 0.1 Penetrant "cubiccentimeters 0.1

Water cubic centimeters 100 Potassium nitrate grams 20 Pyrocatechin do 5Citric acid do 0.5 Thiourea do 0.1 Penetrant cubic centimeters 0.1

Water- ..-cubic centimeters-.. 100 Potassium nitrate grams-- 20Pyrocatechin ..-do---- 5 5 NamPm-rno ...do 0.:

HsPO4 ...-cubic centimeters..- 0.05 Thiourea -grams 0.1 Penetrant cubiccentimeters..- 0.1 Glycerin ..do 5

Water cubic centimeters" 100 Potassium nitrate grams 2o Pyrocatechin do5 mmroi-mo do 0.3

H.1P04 cubic centimeters 0.05 Thiourea grams 0.1 Penetrant -cubiccentimeters 0.1 Glycerin do 5 Pyrocatechin sulfonic acid grams.... 0.5

Water cubic centimeters 100 Potassium nitrate grams- Pyrocatechin--d0---- 5 salicyclic sulfonic acid do 0.3 HaPO4 (85%) cubic centimeters0.05 Thiourea ..grams 0.1 Penetrant cubic centimeters..- 0.1 Glycerin do5 Pyrocatechin sulfonic acid "grams" 0.5

Water -cubic centimeters-.. 100 Potassium nitrate -grams 20 Pyrocatechindo.. 5 NaH2PO4-H2O ..do 0.3 H3PO4 cubic centimeters-.. 0.05 Thioureagrams 0.1 Penetrant "cubic centimeters 0.1

Glycerin do '5 Citric acid "grams..- 0.1

Water "cubic centimeters..- 100 v Potassium nitrate grams-.. 20Pyrocatechin ...do 5 NaH2PO4'H2O ..do 0.3 H3PO4 ..cubic centimeters 0.05Thiourea grams 0.1 Penetrant cubic centimeters- 0.1 Glycerin 5Pyrocatechin sulfonic acid "grams" 0.5 Citric acid on 0.1

Electrolytic recording may be performed on the papers prepared accordingto the above specifications by scanning them with an iron-bearingelectrode carrying modulated signal currents. It

has been found that iron-bearing electrodes containing a hardeningelement are to be preferred. A stainless steel containing about 5 percent molybdenum has been found to be highly satisfactory as therecording electrode.

The present invention has been described and a few specific exampleshave been given. Many combinations will be apparent to those skilled inthe art within the spirit and scope of the invensinamine, hydrazine,phenyl hydrazine, semicarbazide, and their salts, and a chargeneutralizer comprising a. source of trivalent negative ions chosen fromthe class consisting of phosphates, citrates, and naphthol disulfonates.

2. The process of electrolytic recording which comprises passing amodulated current from an electrode consisting essentially of alake-forming metal through a recording medium containing an aqueoussolution of a colorless, lake-forming compound, an acidified solution ofan electrolyte, a charge neutralizer that consists of a water solublecompound furnishing negative ions having a valence of at least three,the current passed being suflicient to oxidize the metal of theelectrode and to cause it to react with the lake-forming compound toform charged colloidal particles, neutralizing the charge, andprecipitating and fixing th color lake in place by means of saidcompound containing the polyvalent negative ion.

3. A medium for electrolytic recording with an electrode consistingessentially of lake-forming metal which comprises a support impregnatedwith an aqueous solution consisting of a. colorless lake-formingcompound which, upon passage of the current becomes a dye and forms anelectrically charged, colloidal lake with the metallic ions fromtheelectrode, a small quantity of a strong reducing agent sufilcient tokeep the background clear, a strong electrolyte to readily conduct anelectric current during recording, and a small quantity of a chargeneutralizer consisting oi a water soluble compound furnishing negativeions of a valence of at least three, the medium being free from mordantforming salts.

4. A medium for electrolytic recording with a lake-forming metalelectrode, including a. support treated with an aqueous solutioncontaining a lake-forming polyhydroxyphenol, a strong electrolyte, ananti-oxidant chosen from the group consisting of thiourea, thiosinamine,hydrazine, phenylhydrazine, semicarbazide and their salts;

and a charge neutralizer comprising a source of trivalent negative ions,chosen from the group consisting of phosphates, naphtholdisulfonates,and citrates.

EDGAR R. WAGNER.

